What Cultivators Need to Know About PAR and PPFD

Gone are the days when you only had to worry about whether a plant should be planted in shade or full sun, and how often to water and feed it. Whether you are a Cannabis cultivator or a home vegetable gardener, you now have far more options to be more efficient. Efficiency improvements can lead to higher yield for same cost. However, all those options also bring in more complexity.

Watering Schedules

Let’s start with optimum watering schedules. You could follow rules of thumb for different species of plants. However, in most cases not only will you be either over or under watering plants at the risk of wasting precious water, but also probably affecting plant health. There are too many variables affecting the water requirements for plants. Those include the plant species, the stage of growth, temperature and humidity of air as well as soil characteristics. The most effective way to determine irrigation schedules is to directly measure the soil moisture levels. Even a few years ago it was quite expensive to deploy such a sensor. Then there is the added complexity of collecting the measurements and translating those into an optimal watering schedule.

With the availability of ubiquitous connectivity and inexpensive sensors, it has now become easy and affordable to deploy such sensors. Not only can you measure soil moisture and compute optimum irrigation schedules easily, you can constantly adjust it as environmental conditions and plant growth stage change.

In an indoor cultivation setting, there are several other factors that affect the health and yield of a plant. These include temperature, relative humidity, CO2 levels, pH level of soil, fertilization, light intensity and light composition. Today we are going to look at one of the more fascinating yet misunderstood areas, measuring light that is helpful for plant growth.

The Mysteries of Light

Several decades ago, scientists understood that only certain bands of sunlight facilitated photosynthesis, the process plants use to synthesize food from light, carbon dioxide and water. The term PAR (Photosynthetically Active Radiation) was coined to designate a spectral range of solar radiation that plants were able to use for photosynthesis. This range was generally understood to be from 400 to 700 nanometers, roughly covering the visible spectrum. This meant that ultraviolet light, and in general, infrared light, had no meaningful contribution to photosynthesis.

Later on some discoveries indicate that near infrared light may have some contribution to photosynthesis. This means that artificial lighting that produced light outside of this band not only contributes nothing to plant growth and yield, but also may harm the plants by producing additional heat.

There are several ways to measure light intensity such as lumens and lux. These are not very effective in measuring the light that contributes to plant growth. PAR describes the wavelength (or quality) of light that is meaningful to plants. But it is not a measurement unit. The amount of PAR light a fixture produces is measured in PPF (Photosynthetic Photon Flux) which has a unit, micro moles per second (µmol/s). The most meaningful term in this context is actually PPFD (Photosynthetic Photon Flux Density), expressed in micro mols per meter squared per second ( µmol/m²/s) which, unlike PPF, is the amount received by a plant.

You can not Improve what you can not Measure

So why should you care about any of these terms? For one, when you choose a lighting fixture for indoor cultivation, it is important to know the efficiency of the fixture. The normal units typically used in lighting such as lumens or watts don’t indicate the plant growing efficiency of these plants. What really determines the efficiency of such fixtures is how much Photons it can produce in the PAR region.

But PPF/PPFD is not used only for selecting a lighting fixture. Knowing how much PPFD a plant is receiving will go a long way in determining the yield that plant will produce. Even with lights with the same output, distance from plants can change the PPFD value at the foliage. A very high PPFD value may not result in proportional improvement in yield. But it may produce excess heat and use up too much energy.

Plant Requirements

The PPFD requirement of plants can also change with the growth stage of the plant. The approximate ranges of PPFD that is suitable for plants at different growth stage are:

An interesting observation here is that if you have a light fixture that can produce 900 PPFD for a plant at the flowering stage, it is not necessary to turn it on full blast while the plant is not flowering yet. By doing so you can save electricity costs and reduce stress on the plant.

Additionally, if supplemental CO2 is used, plants can often utilize higher levels of PPFD.

So how does all this help in practical terms, besides the one time calculation needed when buying and operating light fixtures?

Practical Solutions

Constant monitoring of environmental conditions such as soil moisture, temperature, humidity, CO2 levels and PPFD can help optimize grow conditions. Until recently only instrumentation labs were able to provide sensors that could measure all these conditions. They came at a very high cost, and even then continuous measurement was labor intensive. Also, translating these observations to practical recommendations were often very difficult.

Entrinsia is one of the first companies to introduce a set of very inexpensive sensors. These sensors can continuously monitor the grow room conditions and translate the readings to actionable inputs. These sensors and apps can give you continuous readings of these sensors and daily summaries. It will either confirm that the plants are receiving adequate PPFD and for sufficient duration, or that there are issues to address. Over time it can also correlate these conditions with yield, providing a means to continuously improve the grow conditions. All the readings are collected and uploaded to your cloud account continuously, allowing you to access them using your smart phone or web account. You also receive notifications when there are conditions that require attention, as well as suggestions for fine tuning the grow conditions.

The best news is that most of these sensors cost little more than a cup of coffee, per month!

To learn more and see how inexpensive and easy it is to get started, click here